I think this was my error.
I had edited theme-functions.php and accidentally added a space after the final “>”.
This sometimes resulted in a blank page but sometimes the following error message which pointed me to the source of the problem
Warning: Cannot modify header information – headers already sent by (output started at /home/compton/public_html/web/wp-content/themes/graphene/includes/theme-functions.php:279) in/home/compton/public_html/web/wp-includes/pluggable.php on line 866